It is pretty much possible that you would have to maintain and evolve a legacy project, based on an existing database, with a lot of already written SQL statements - see Legacy code and existing projects.
For instance, you would like to use mORMot for new features, and/or
add mobile or HTML clients - see Cross-Platform
In this case, the ORM advanced features - like ORM Cache or BATCH process, see BATCH sequences for adding/updating/deleting records - may conflict with the legacy code, for the tables which may have to be shared.
Here are some guidelines when working on such a project.
To be exhaustive about your question, we need to consider each ORM CRUD
We may have to divide them in three kinds: read queries, insertions, and modifications of existing data.